Method of handling radio link failure in wireless communication system based on multi-eNB connectivity and apparatus thereof

ABSTRACT

Methods and apparatuses are provided in a wireless communication system supporting dual connectivity of a first base station and a second base station. A predetermined number of consecutive out-of-syncs for a special cell are received. A timer for the special cell is started. A radio link failure (RLF) is detected upon an expiration of the timer. A failure message including failure type is transmitted to the first base station in case that the RLF for the special cell is associated with the second base station.

PRIORITY

This application is a Continuation application of U.S. patentapplication Ser. No. 16/255,123, filed in the U.S. Patent and TrademarkOffice (USPTO) on Jan. 23, 2019, which is a Continuation Application ofU.S. patent application Ser. No. 15/158,157, filed in the USPTO on May18, 2016, now U.S. Pat. No. 10,225,784, issued on Mar. 5, 2019, which isa Continuation Application of U.S. patent application Ser. No.14/457,898, filed in the USPTO on Aug. 12, 2014, now U.S. Pat. No.9,723,533, issued on Aug. 1, 2017, which claims priority under 35 U.S.C.§ 119(a) to Indian Provisional Patent Application Nos. 948/KOL/2013 and361/KOL/2014, filed on Aug. 12, 2013 and Mar. 21, 2014, respectively,and Korean Patent Application Nos. 10-2013-0136482 and 10-2014-0050293,filed on Nov. 11, 2013 and Apr. 25, 2014, respectively, the contents ofwhich are incorporated herein by reference.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates generally to a communication method andapparatus between a User Equipment (UE) and an evolved NodeB (eNB) in awireless communication system, and more particularly, to a method ofhandling multi-eNB connectivity when a radio link failure of the UE isgenerated.

2. Description of the Prior Art

A 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE)(Release 11 and earlier) system is based on a single connectivity modelin which one UE is connected to one eNB. In such a system, handlingRadio Link Failure (RLF) is based on the detection of the RLF by the UE.A link with the eNB is in an RLF state, so that the RLF is based onadditional actions performed by the UE without any notification of theRLF to the eNB.

FIG. 1 is a diagram illustrating handling of the RLF in a mobilecommunication system, and FIG. 2 is a flowchart illustrating a processof handling the RLF in a mobile communication system.

Referring to FIG. 1, in order to detect the RLF, the UE may perform aprocess known as Radio Link Monitoring (RLM). The UE may measure a BlockError Rate (BLER) of a Physical Downlink Control Channel (PDCCH) duringa predetermined time period. When the BLER drops below a presetthreshold (Qout) during the predetermined time period, an out-of-syncindication may be generated in a physical (PHY) layer. When a presetN310 number of successive out-of-sync indications are reported to aRadio Resource Control (RRC) layer by the PHY layer, an RLF timer T310is started by the RRC layer. At this time, when the RLF timer expires,the RLF may be declared. Further, when the timer T310 starts, and then apredefined N311 number of successive in-sync indications are reported bythe PHY layer, the timer T310 may stop.

Referring to FIG. 2, a UE 210 generates the RLF in a first link, in step251. Further, after the timer T310 expires, the UE 210 may declare theRLF. In the declaration of the RLF, the UE 210 may stop all uplinktransmission to avoid a possibility of the generation of uplinkinterference, and deactivate all radio bearers. Further, the UE 210scans and is synchronized with a target cell 240, in step 252, andtransmits a connection re-establishment request message to the targetcell 240, in step 253. When the target cell 240 already has UE context,in step 254, the target cell 240 transmits a connection re-establishmentsuccess message to the UE 210, in step 255. After the successfulre-establishment, the connection between the UE 210 and the target cell240 resumes, in step 256.

However, when the target cell 240 does not have the UE context, in step257, the target cell 240 transmits a connection re-establishment failuremessage to the UE 210, in step 258. After the connection failure, the UE210 enters an idle mode, in step 259, and the UE 210 eventually triggersa new connection with the target cell 240, in steps 260 and 261. Thismay cause an application level connection to terminate, and thus, isvery undesirable. Further, it should be noted that the target cell 240has the UE context only when a handover is performed before the RLF isgenerated. Accordingly, a re-establishment process may be successfullyachieved only in such a scenario.

When the preset maximum number of random accesses has failed, theexistence of the system RLF may be also triggered. Further, when thepreset maximum number of RLF retransmissions is performed, the RLF maybe also triggered.

The related art discloses content in which the UE scans a proper targetcell after declaring the RLF, and transmits an RLF indication indicatingthat the RLF is generated in a previous eNB (by notifying of an ID ofthe previous eNB) to the selected target cell in a single connectivitysystem (3GPP LTE Release 11 and earlier). The target cell transmits theinformation to the previous eNB. The previous eNB having received theinformation may improve handover parameters, which may cause the RLF andother radio link parameters.

The terms eNB and cell may be exchangeably used throughout.

SUMMARY OF THE INVENTION

The present invention has been made to address at least the aboveproblems and/or disadvantages and to provide at least the advantagesdescribed below. Accordingly, an aspect of the present invention amechanism which can efficiently handle the RLF and reduce an influenceon an application level connection in a system where the UE is connectedto one or more eNBs.

In accordance with an aspect of the present invention, a method by aterminal is provided in a wireless communication system supporting dualconnectivity of a first base station and a second base station. Apredetermined number of consecutive out-of-syncs for a special cell arereceived. A timer for the special cell is started. An RLF is detectedupon an expiration of the timer. A failure message including failuretype is transmitted to the first base station in case that the RLF forthe special cell is associated with the second base station.

In accordance with another aspect of the present invention, method by afirst base station is provided in a wireless communication systemsupporting dual connectivity of a first base station and a second basestation. A terminal is connected with via a cell associated with thefirst base station. An RLF for a special cell is detected upon anexpiration of a timer for the special cell. A failure message includingfailure type is received from the terminal in case that the RLF for thespecial cell is associated with the second base station. The timer isstarted in case that a predetermined number of consecutive out-of-syncsfor the special cell is received at the terminal.

In accordance with another aspect of the present invention, a terminalis provided in a wireless communication system supporting dualconnectivity of a first base station and a second base station. Theterminal includes a transceiver, and a controller coupled with thetransceiver. The controller is configured to receive a predeterminednumber of consecutive out-of-syncs for a special cell, start a timer forthe special cell, detect an RLF upon an expiration of the timer, andtransmit a failure message including failure type to the first basestation in case that the RLF for the special cell is associated with thesecond base station.

In accordance with another aspect of the present invention, a first basestation is provided in a wireless communication system supporting dualconnectivity. The first base station includes a transceiver, and acontroller coupled with the transceiver. The controller is configured toconnect with a terminal via a cell associated with the first basestation. An RLF for a special cell is detected upon an expiration of atimer for the special cell. The controller is also configured toreceive, from the terminal, a failure message including failure type incase that the RLF for the special cell is associated with the secondbase station. The timer is started in case that a predetermined numberof consecutive out-of-syncs for the special cell is received at theterminal.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects, features, and advantages of the presentinvention will be more apparent from the following detailed descriptionwhen taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a diagram illustrating the handling of RLF in a mobilecommunication system;

FIG. 2 is a flowchart illustrating a process of handling the RLF in amobile communication system;

FIG. 3 is a diagram illustrating dual connectivity, according to anembodiment of the present invention;

FIG. 4 is a flowchart illustrating a method of handling the RLF in dualconnectivity, according to an embodiment of the present invention;

FIG. 5 is a flowchart illustrating a method of handling the RLF in dualconnectivity, according to another embodiment of the present invention;

FIG. 6 is a flowchart illustrating a method of handling the RLF in dualconnectivity, according to another embodiment of the present invention;

FIG. 7 is a flowchart illustrating a method of handling the RLF of FIG.6 when a link of a Master eNB (MeNB) corresponds to the RLF, accordingto an embodiment of the present invention;

FIG. 8 is a flowchart illustrating a method of handling the RLF of FIG.6 when a link of a Secondary eNB (SeNB) corresponds to the RLF,according to an embodiment of the present invention;

FIG. 9 is flowchart illustrating a process of an early indication of theRLF in a single connectivity system;

FIG. 10 is a flowchart illustrating a method of switching to singleconnectivity from dual connectivity, according to an embodiment of thepresent invention;

FIG. 11 is a flowchart illustrating a method of switching to singleconnectivity from dual connectivity, according to another embodiment ofthe present invention;

FIG. 12 is a flowchart illustrating a method of switching to singleconnectivity from dual connectivity, according to another embodiment ofthe present invention;

FIG. 13 is a flowchart illustrating a method of switching to singleconnectivity from dual connectivity, according to another embodiment ofthe present invention;

FIG. 14 is a flowchart illustrating a method of switching to dualconnectivity from single connectivity, according to an embodiment of thepresent invention;

FIG. 15 is a flowchart illustrating a method of switching to dualconnectivity from single connectivity, according to another embodimentof the present invention;

FIG. 16 is a block diagram illustrating a UE, according to an embodimentof the present invention;

FIG. 17 is a block diagram illustrating an eNB, according to anembodiment of the present invention;

FIG. 18 is a flowchart illustrating an operation of transmitting an RLFindication by a UE and an MeNB, according to an embodiment of thepresent invention;

FIG. 19 is a flowchart illustrating the operation of transmitting theRLF indication by the UE and the MeNB, according to another embodimentof the present invention;

FIG. 20 is a flowchart illustrating a method of transmitting an RLFindication including an RLF cause value, according to an embodiment ofthe present invention;

FIG. 21 is a flowchart illustrating a method of transmitting an RLFindication included in a measurement report configuration, according toan embodiment of the present invention;

FIG. 22 is a flowchart illustrating the method of transmitting the RLFindication included in the measurement report configuration, accordingto another embodiment of the present invention; and

FIGS. 23 to 25 are diagrams illustrating MAC CE, according to anembodiment of the present invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS OF THE PRESENT INVENTION

Embodiments of the present invention are described in detail withreference to the accompanying drawings. The same or similar componentsmay be designated by the same or similar reference numerals althoughthey are illustrated in different drawings. Detailed descriptions ofconstructions or processes known in the art may be omitted to avoidobscuring the subject matter of the present invention.

The terms described below are defined in consideration of the functionsin the present disclosure, and may be different according to users,intentions of the users, or customs. Therefore, the definitions of theterms should be determined based on the contents throughout thespecification.

FIG. 3 is a diagram illustrating dual connectivity, according to anembodiment of the present invention.

Referring to FIG. 3, 3GPP LTE Release 12 has discussed dual connectivityas a core technology to increase user efficiency and achieve highmobility robustness. In such a dual connectivity scenario, the UE may besimultaneously connected to a macro cell and a pico cell. However,simultaneous transmission/reception of the two cells may be or may notbe possible according to the capability of the UE. The macro cell andthe pico cell are in harmony to be helpful to the UE. In such a system,the macro cell serves as a mobility anchor and hides a pico cell changefrom a core network, which can significantly reduce signaling loads ofthe core network. While the pico cell may handle much of the traffic,the macro cell may handle more important traffic such as, for example,Voice over Internet Protocol (VoIP) traffic. One cell (macro cell) mayserve as an MeNB and the other cell (pico cell) may serve as an SeNB.

At this time, it is preferable to maintain as many UEs as possible in adual connectivity mode during an RRC connected state in order to keephigh user efficiency.

The RLF in one link may cause a loss of the dual connectivity mode, andaccordingly interrupt the connectivity. Accordingly, in handing the LRFin the dual connectivity, it is important for the UE to maintain thedual connectivity mode even through one of the radio links is in an RLFstate.

Handling of the RLF in the dual connectivity system is described ingreater detail below.

In the dual connectivity, one or both of an MeNB link and an SeNB linkmay be in the RLF state. As described below, when one link is in the RLFstate, it may be referred to as “single link RLF” or “partial RLF”.Further, when both links are in the RLF state, it may be referred to as“dual link RLF” or “complete RLF”. As described above, the RLF scenarioincludes the following:

1. Single link RLF

-   -   a. MeNB link is in RLF state    -   b. SeNB link is in RLF state

2. Dual link RLF

-   -   a. Both MeNB link and SeNB link are in RLF state

Since the UE has two links (one is the link with the MeNB and the otherone is the link with the SeNB) in the dual connectivity, the UE may haveone good link even though the other link is in the RLF state, therebyimproving the handling of the RLF. That is, the good link may be usedfor handling the RLF of the other link.

In Carrier Aggregation (CA) of 3GPP LTE Rel. 10/11, a Primary Cell(Pcell) may determine Secondary Cell (Scell) link quality based on aChannel Quality Indicator (CQI) report. Further, since it is assumedthat an SCell can be deactivated according to the SCell link quality,RLM for the SCell is not performed. CQIs of all cells are configured tobe reported to the PCell through only a Physical Uplink Control CHannel(PUCCH) through which the CQIs are transmitted. However, whether theMeNB can have a CQI report of the SeNB in the dual connectivity has notbeen yet agreed (scenario 1b). Further, when a connection between theMeNB and the UE does not have good radio quality, it is unclear how theMeNB has its CQI (scenario 1a).

In addition, in the CA of Rel. 10/11, the PUCCH is transmitted only inthe PCell. However, when the same access cannot be made in the dualconnectivity, non-ideal backhaul may influence the operation.

In the CA of Rel. 10/11, the handling of the RLF is applied only whenthe MeNB (PCell) link is in the RLF state, and may refer to there-establishment for all connections including the connections of theSeNB (SCell). When the re-establishment is not successful, allconnections are terminated and may be required to be re-established.Such access may consider the SeNB link to be good while it is still inthe RLF.

A re-establishment process may be successful only when a cell in whichthe re-establishment is performed has been already prepared to acceptthe UE. A situation that is not a handover may not influence theconnection being progressed.

In contrast, there has not been a process for handling the RLF of theSeNB (SCell).

In a method of handling the RLF, the RLF for a link in the RLF state maybe independently performed. For example, in the scenario 1b, after theRLF for the SeNB is declared, the UE may find a suitable target cell andattempt the re-establishment. When the target cell already has the UEcontext, the re-establishment may be successful. When the target celldoes not already have the UE context, connections operating for the linkof the SeNB (eNB having a link in which the RLF is generated) will beremoved and initial connection establishment for the connections shouldbe made (as initial dual connectivity mode establishment). Such a methodis suitable for the scenario 1b, and may be also applied to the scenario1a.

Alternatively, when the re-establishment fails, the UE may be switchedto a single connectivity with an eNB having a good link.

The above described methods of handling the RLF may have a bad influenceon application level connections, and accordingly, may increase the lossof the dual connectivity mode and the switching between the dualconnectivity and the single connectivity.

In an embodiment of the present invention, a method is described inwhich a good link (which is not in the RLF state) informs the MeNB ofthe RLF state of another link. The MeNB may prepare an alternative cellinstead of the cell of the link in the RLF state. Thereafter, the MeNBmay perform triggering such that the UE hands over to the prepared cellinstead of performing re-establishment. This may prevent the loss of thedual connectivity mode and maintain continuous connections in a smoothmanner. Alternatively, when the suitable alternative cell is not found,the MeNB may prepare a cell having a good link to provide the singleconnectivity mode to the UE (from cells in connection with the provisionof the dual connectivity to the UE). In order to detect the RLF from thelink, an RLM process may be performed on the link.

The above-described method may be applied in both the RLF scenario 1aand scenario 1b. At this time, the UE context is required to bemaintained until a predetermined time after the RLF timer T310 expires.Further, in order to quickly prepare the alternative cell, an RLFindication may be transmitted to the MeNB through another good linkbefore the RLF is declared in one link.

FIG. 18 is a flowchart illustrating an operation of transmitting an RLFindication by the UE and the MeNB, according to an embodiment of thepresent invention.

Referring to FIG. 18, a UE 1810 is dually connected to an MeNB 1820 andan SeNB 1830, in step 1851. Further, in step 1852, the UE 1810 performsthe RLM in one or more cells of the MeNB 1820 and the SeNB 1830.Thereafter, when the UE 1810 detects the RLF in the cell of the SeNB1830, in step 1853, the UE 1810 transmits an RLF indication, includingRLF cause information, to the PCell of the MeNB 1820, in step 1854.After transmitting the RLF indication, the UE 1810 stops all uplinktransmission including the configured PUCCH, in step 1855, and stops adata radio bearer which is working in the cell from which the RLF isdetected, in step 1856.

FIG. 19 is a flowchart illustrating the operation of transmitting theRLF indication by the UE and the MeNB, according to another embodimentof the present invention.

Referring to FIG. 19, a UE 1910 is dually connected to an MeNB 1920 andan SeNB 1930, in step 1951. Further, the UE 1910 performs the RLM in aPCell of the MeNB 1920 and one cell of the SeNB 1930 configured as apathloss reference. The cell configured as the pathloss reference may bereferred to as a pSCell of the SeNB 1930, or a special cell. When the UE1910 detects the RLF in the pSCell, in step 1953, the UE 1910 transmitsan RLF indication, including RLF cause information, to the PCell of theMeNB 1920, in step 1954. After transmitting the RLF indication, the UE1910 stops all uplink transmission including the configured PUCCH, instep 1955, and stops a data radio bearer, which is working in the cellfrom which the RLF is detected, in step 1956. In some embodiments of thepresent invention, the PUCCH may be configured in the cell that has beenconfigured as the pathloss reference.

According to another embodiment of the present invention, the RLFindication for one link may be transmitted to the MeNB through the othergood link at the following time points:

1. Before RLF is declared (before T310 expires)

-   -   a. RLF expected indication

2. When RLF is declared (when T310 expires)

-   -   a. RLF expected indication

When the indication for the RLF state is received, the MeNB may preparean alternative cell, which can provide a service to the UE on behalf ofa cell that is expected to have the RLF (or which has the alreadygenerated RLF). Accordingly, a connection re-establishment can beprevented.

FIG. 4 is a flowchart illustrating a method of handling the RLF in thedual connectivity, according to an embodiment of the present invention.

Referring to FIG. 4, when the RLF is declared in the dual connectivity,the dual connectivity may be replaced with the single connectivity to aneNB having a good link.

When the RLF of one of the two radio links is declared (for example,when the timer T310 expires), a result thereof may be notified to theMeNB through a second link (that is, a link having a good connectionstate). Further, the MeNB may prepare an eNB having a good link toprovide a service to the UE in the single connectivity mode and operatethe UE to perform switching to the single connectivity mode.

Specifically, the RLF for the first link is generated, in step 451. Forexample, when the timer T310 expires, the RLF may be declared.

Thereafter, in step 452, a UE 410 transmits an RLF message to an MeNB420. At this time, the RLF message may be transmitted to the MeNB 420through the second link having a good connection state.

In some embodiments of the present invention, the UE 410 may transmit amessage, including an indication indicating that the RLF has beenalready generated, to the MeNB 420. At this time, in some embodiments ofthe present invention, the RLF indication may be defined as a newindication.

Alternatively, in some embodiments of the present invention, the UE 410may transmit a legacy message RRC connection re-establishment message tothe MeNB 420 through the second link. The RRC connectionre-establishment message may be the same as that transmitted in 3GPP LTEwhen the timer T310 expires. For example, a Random Access CHannel (RACH)code may be given to the UE 410 for such a purpose. At this time, thetransmitted RRC connection re-establishment message may be dealt withlike the RLF indication.

As illustrated in step 450, L2/L3 context for the first link may bemaintained for a preset period. The L2/L3 context is required to bemaintained for a predetermined period to prepare the single connectivityreplacement. Then, the L1 context for the first link may be released.

Thereafter, in step 453, the MeNB 420 communicates with an SeNB 430 tohave the UE 410 ready to switch to the single connectivity between theUE 410 and an eNB having a good link.

For example, while the radio link of the MeNB 420 is in a good state,the radio link of the SeNB 430 may be in an RLF state. At this time, theMeNB 420 may allow the UE 410 to switch to the single connectivity withthe MeNB 420, instead of the dual connectivity with both the MeNB 420and the SeNB 430. At this time, context of the flow serviced by the SeNB430 is required to be transferred to the MeNB 420. A detaileddescription thereof is set forth below.

The MeNB 420 operates to switch to the single connectivity, in step 454,and transmits a single connectivity switching message to the UE 410, instep 455. At a this time, flow information changed in step 453 may bealso transferred.

Thereafter, the UE 410 performs switching to the single connectivitywith the eNB having a good link, in step 456.

In an embodiment of the present invention described in connection withFIG. 4, the handling of the RLF in the link of the SeNB 430 may bedelayed, and thus, the link of the SeNB 430 is in the RLF state. Thisembodiment of the present invention may be suitable for the RLF scenario1b corresponding to a case where the link of the MeNB 420 is good, butis not limited thereto.

FIG. 5 is a flowchart illustrating the handling method when the RLF isgenerated in the dual connectivity, according to another embodiment ofthe present invention.

Referring to FIG. 5, a UE 510 transmits an RLF message to an MeNB 520through a good link after the RLF timer T310 expires to enable the MeNB520 to prepare an alternative cell for providing a service to the UE 510in the dual connectivity.

The MeNB 520, having received the RLF message, identifies whether thereis a proper target cell 540 that can be prepared for a handover. Whenthere is no proper target cell 540, the MeNB 520 determines and preparesthe proper target cell 540 for the handover by performing propermeasurement. Further, the MeNB 520 instructs the UE 510 to hand over tothe prepared proper target cell 540.

When the MeNB 520 finds no proper target cell for the handover, the MeNB520 prepares an eNB corresponding to a good link, through which aservice is provided, to the UE 510 in the single connectivity mode.Further, the MeNB 520 may instruct the UE 510 to switch to the singleconnectivity mode with the selected eNB.

Specifically, in step 551, the RLF for a first link is generated. Forexample, when the timer T310 expires, the RLF may be declared.

Thereafter, in step 552, the UE 510 transmits an RLF message to the MeNB520. At this time, the RLF message may be transmitted to the MeNB 520through a second link having a good connection state. In order toperform the handover or prepare switching to the single connectivity, itis required to retain the L2/L3 context for the first link for a presettime period and a new timer, such as a context retention timer, isneeded to retain the L2/L3 context. Meanwhile, the L1 context for thefirst link is released.

In some embodiments of the present invention, the UE 510 may transmit amessage including an indication indicating that the RLF has been alreadygenerated to the MeNB 520. At this time, the RLF indication may bedefined as a new indication.

Alternatively, in some embodiments of the present invention, the UE 510may transmit a legacy message RRC connection re-establishment message tothe MeNB 520 through the second link. The RRC connectionre-establishment message may be the same as that transmitted in 3GPP LTEwhen the timer T310 expires. For example, an RACH code may be given tothe UE 510 for such a purpose. The transmitted RRC connectionre-establishment message may be dealt with like the RLF indication.Accordingly, the RRC connection re-establishment message correspondingto the RLF indication may alternatively identify a link in which the RLFis generated.

Thereafter, when the MeNB 520 does not have an effective recentmeasurement report from the UE 510, the MeNB 520 transmits a measurementinstruction to the UE 510, in step 553. However, as illustrated in step554-1, for example, when the MeNB 520 already has the still effectiverecent measurement report from the UE 510, the measurement may not berequired. In step 554, the UE 510 transmits the measurement report tothe MeNB 520 by performing the measurement according to the measurementinstruction of the MeNB 520.

At this time, in some embodiments of the present invention, when theMeNB 520 receives the RLF indication, the MeNB 520 may configure a oneshot measurement. Alternatively, in some embodiments of the presentinvention, when the MeNB 520 receives the RLF indication, the MeNB 520may configure a one shot measurement if the first link is a macro link,and may configure normal measurement if the first link is not the macrolink.

The MeNB 520 receives the measurement report from the UE 510, in step554. Alternatively, when the MeNB 520 already has the effectivemeasurement report, the MeNB 520 prepares a cell for the handover, instep 555. At this time, the MeNB 520 prepares a target eNB 540 suitablefor the handover by the UE 510 based on the measurement report. Thetarget eNB 540 may be a pico eNB but is not limited thereto, and may bea macro eNB.

Meanwhile, when the target cell suitable for the handover is not foundas illustrated in step 556-1, the MeNB 520 communicates with an SeNB 530to have the UE 510 ready to switch to the single connectivity betweenthe UE 510 and an eNB having a good link, in step 556.

At this time, in some embodiments of the present invention, the MeNB 20may select one target cell 540 suitable for the handover by the UE 510from among a plurality of cells based on the measurement report.However, when the suitable cell is not found, the MeNB 520 may prepare asecond eNB for the single connectivity mode. For example, while theradio link of the MeNB 520 is in a good state, the radio link of theSeNB 530 may be in an RLF state. At this time, the MeNB 520 may allowthe UE 510 to switch to the single connectivity with the MeNB 520,instead of the dual connectivity with both the MeNB 520 and the SeNB530. Context of the flow serviced by the SeNB 530 is required to betransferred to the MeNB 520. A detailed description thereof is set forthbelow.

Alternatively, in some embodiments of the present invention, the MeNB520 may select one target cell 540 suitable for the handover by the UE510 from among a plurality of cells based on the measurement report.However, when the suitable cell is not found, the MeNB 520 may preparean alternative cell for the handover. The alternative cell may besemi-statically determined by the MeNB 520 and/or the SeNB 530.Alternatively, in some embodiments of the present invention, thealternative cell may be dynamically designated by a first eNB as theMeNB 520. Thereafter, when no cell is configured as the alternativecell, the MeNB 520 may prepare a second eNB for the single connectivitymode. At this time, as described above, the context of the flow servicedby the first eNB is required to be transferred to the second eNB. Adetailed description thereof is set forth below.

When the MeNB 520 prepares the target cell 540 suitable for the handoverof step 555, in step 557, the MeNB 520 instructs the UE 510 to hand overto the prepared target cell 540, in step 558. For example, the MeNB 520may transmit a handover instruction to the UE 510. At this time, thehandover instruction may be transmitted through the second link having agood connection state. In some embodiments of the present invention, theprepared target cell 540 may be at least one of a cell from themeasurement report transmitted by the UE 510, an alternative cell, andsecond eNBs having good links.

Meanwhile, when the MeNB 520 does not select the target cell 540suitable for the handover, the MeNB 520 operates to switch to the singleconnectivity with the second eNB, in step 559, and transmits a singleconnectivity switching message to the UE 510, in step 560. At this time,flow information changed in step 556 may be also transferred.

In an embodiment of the present invention described in connection withFIG. 5, the handling of the RLF in the link of the SeNB 530 may bedelayed, and thus, the link of the SeNB 530 is in the RLF state, and theembodiment may be suitable for the RLF scenario 1b corresponding to acase where the link of the MeNB 520 is good, but is not limited thereto.

FIG. 6 is a flowchart illustrating a method of handling the RLF in thedual connectivity, according to another embodiment of the presentinvention.

Referring to FIG. 6, a UE 610 transmits an RLF expected message to anMeNB 620 through a link having a good link connection before the RLFtimer T310 expires to enable the MeNB 620 to prepare an alternative cellfor providing a service to the UE 610 in the dual connectivity.

The RLF expected message may be transmitted at the time when the RLFtimer T310 starts or after a preset time from the start of the RLF timerT310 to secure some time during which initial RLF is restored.

The MeNB 620, having received the RLF expected message, identifieswhether there is the suitable target cell 640 that can be prepared forthe handover. When there is no suitable target cell 640, the MeNB 520determines and prepares the suitable target cell 640 for the handover byperforming proper measurement. Thereafter, when the RLF is actuallygenerated (for example, when the timer T310 expires), the UE 610explicitly informs of the generation of the RLF by transmitting amessage, such as, for example, the RLF message, to the MeNB 620 througha link having a good link connection. Further, the MeNB 620 instructsthe UE 610 to hand over to the prepared target cell 640 based on thedetection of the RLF.

When the MeNB 620 finds no suitable target cell 640 for the handover,the MeNB 620 prepares an eNB corresponding to a good link through whicha service is provided to the UE 610 in the single connectivity mode.Further, the MeNB 620 instructs the UE 610 to switch to the singleconnectivity mode with the selected eNB based on the detection of theRLF.

Specifically, in step 651, the RLF for a first link is generated. Forexample, when the RLF timer T310 starts, the RLF may be expected.

Thereafter, in step 652, the UE 610 transmits the RLF expected messageto the MeNB 620. At this time, the RLF expected message may betransmitted to the MeNB 620 through a second link having a goodconnection state.

In some embodiments of the present invention, the UE 610 may transmit amessage including an RLF expected indication indicating that the RLF isexpected to the MeNB 620. At this time, in some embodiments of thepresent invention, the RLF expected indication may be defined as a newindication.

In some embodiments of the present invention, when the timer T310starts, the UE 610 may transmit the RLF expected message for the firstlink to the MeNB 620.

Alternatively, in some embodiments of the present invention, when thetimer T310 starts, an RLF preparation timer corresponding to a new timermay start. Further, when the RLF preparation timer expires, the UE 610may transmit the RLF expected message for the first link to the MeNB620. A length of time of the RLF preparation timer may be shorter than alength of time of the RLF timer T310. Alternatively, when the RLF timerstarts, if out-of-sync indications are continuously reported for apreset time, the UE 610 may transmit the RLF expected message to theMeNB 620.

Further, in some embodiments of the present invention, a threshold of anew Reference Signal Received Power (RSRP) may be defined as an RLFreport threshold. At this time, when the RLF report threshold issatisfied after the timer T310 starts, the UE 610 may transmit the RLFexpected message for the first link to the MeNB 620.

Alternatively, in some embodiments of the present invention, hysteresismay be defined based on the RLF preparation timer and the RLF reportthreshold. At this time, when an average of the RSRPs for the RLFpreparation timer remains below the RLF report threshold, the UE 610 maytransmit the RLF expected message for the first link to the MeNB 620.

Further, in some embodiments of the present invention, CQI reports ofthe first link may be transmitted on the second link and serve asindications of the RLF state. The MeNB 620 and an SeNB 630 may recognizethe RLF state based on the reports.

When the MeNB 620 does not have an effective recent measurement reportfrom the UE 610 after receiving the RLF expected message for a relatedlayer, the MeNB 620 transmits a measurement instruction to the UE 610,in step 653. However, as illustrated in step 654-1, for example, whenthe MeNB 620 already has the still effective recent measurement reportfrom the UE 610, the measurement may not be required. In step 654, theUE 610 transmits the measurement report to the MeNB 620 by performingthe measurement according to the measurement instruction of the MeNB620.

At this time, in some embodiments of the present invention, when theMeNB 620 receives the RLF expected indication, the MeNB 620 mayconfigure a one shot measurement. Alternatively, in some embodiments ofthe present invention, when the MeNB 620 receives the RLF expectedindication, the MeNB 620 may configure a one shot measurement when thefirst link is a macro link and may configure normal measurement when thefirst link is not the macro link.

The MeNB 620 receives the measurement report from the UE 510, in step654. Alternatively, when the MeNB 520 already has the effectivemeasurement report, the MeNB 520 prepares a cell for the handover, instep 655. At this time, the MeNB 620 prepares the target cell 640suitable for the handover by the UE 610 based on the measurement report.The target cell 640 may be a macro eNB but does not have to be limitedthereto, and may be a pico eNB.

Meanwhile, when the target cell suitable for the handover is not foundas illustrated, in step 656-1, the MeNB 620 communicates with the SeNB630 to have the UE 610 ready to switch to the single connectivitybetween the UE 610 and an eNB having a good link, in step 656.

At this time, in some embodiments of the present invention, the MeNB 620may select one target cell 640 suitable for the handover by the UE 610from among a plurality of cells based on the measurement report.However, when the suitable cell is not found, the MeNB 620 may prepare asecond eNB for the single connectivity mode. For example, while theradio link of the SeNB 630 is in a good state, the radio link of theMeNB 620 may be in an RLF state. At this time, the MeNB 620 may allowthe UE 610 to switch to the single connectivity with the SeNB 630,instead of the dual connectivity with both the MeNB 620 and the SeNB630. At this time, context of the flow serviced by the MeNB 620 isrequired to be transferred to the SeNB 630. A detailed descriptionthereof is set forth below.

Alternatively, in some embodiments of the present invention, the MeNB620 may select one target cell 640 suitable for the handover by the UE610 from among a plurality of cells based on the measurement report.However, when the suitable cell is not found, the MeNB 620 may preparean alternative cell for the handover. The alternative cell may besemi-statically determined by the MeNB 620 and/or the SeNB 630.Alternatively, in some embodiments of the present invention, thealternative cell may be dynamically designated by a first eNB as theMeNB 620. Thereafter, when no cell is configured as the alternativecell, the MeNB 620 may prepare a second eNB for the single connectivitymode. At this time, as described above, the context of the flow servicedby the first eNB is required to be transferred to the second eNB. Adetailed description thereof is set forth below.

In step 657, the RLF is declared. That is, the timer T310 expires andthe RLF may be declared.

Thereafter, in step 658, the UE 610 transmits an RLF message to the MeNB620. At this time, the RLF message may be transmitted to the MeNB 620through a second link having a good connection state.

In some embodiments of the present invention, the UE 610 may transmit amessage including an indication indicating that the RLF has been alreadygenerated to the MeNB 620. At this time, in some embodiments of thepresent invention, the RLF indication may be defined as a newindication.

Alternatively, in some embodiments of the present invention, the UE 610may transmit a legacy message Radio RRC connection re-establishmentmessage to the MeNB 620 through the second link. The RRC connectionre-establishment message may be the same as that transmitted in 3GPP LTEwhen the timer T310 expires. For example, an RACH code may be given tothe UE 610 for such a purpose. At this time, the transmitted RRCconnection re-establishment message may be dealt with like the RLFindication. Alternatively, the MeNB 620 may determine that the RLF isgenerated in the first link, not the second link through which the RRCconnection re-establishment message is received. Accordingly, the RRCconnection re-establishment message corresponding to the RLF indicationmay alternatively identify the link in which the RLF is generated.

At this time, the L2/L3 context is required to be maintained for apredetermined period to prepare the handover or the single connectivityreplacement.

When the MeNB 620 prepares a target cell 640 suitable for the handover,in step 659, the MeNB 620 instructs the UE 610 to hand over to theprepared target cell 640, in step 660. For example, the MeNB 620 maytransmit a handover instruction to the UE 610. At this time, thehandover instruction may be transmitted through the second link having agood connection state. In some embodiments of the present invention, theprepared target cell 640 may be at least one of a cell from themeasurement report transmitted by the UE 610, an alternative cell, andsecond eNBs having good links.

Meanwhile, when the MeNB 620 does not select the target cell 640suitable for the handover, the MeNB 620 operates to switch to the singleconnectivity with the second eNB, in step 661, and thus, transmits asingle connectivity switching message to the UE 610, in step 662. Atthis time, flow information changed in step 656 may be also transferred.

In an embodiment of the present invention described in connection withFIG. 6, the handling of the RLF in the link of the MeNB 620 may bedelayed, and thus, the link of the MeNB 620 is in the RLF state, and theembodiment may be suitable for the RLF scenario 1a corresponding to acase where the link of the SeNB 630 is good but is not limited thereto.

FIG. 7 is a flowchart illustrating a method of handling the RLF of FIG.6 when a link of the MeNB corresponds to the RLF, according to anembodiment of the present invention.

Referring to FIG. 7, in step 701, the UE determines whether the timerT310 of the MeNB link starts. When the timer T310 of the MeNB starts,the UE determines whether the timer T310 of the SeNB does not operate,in step 703. As a result of the determination, when the link of the SeNBis good, an RLF preparation timer starts, in step 705. Thereafter, it isdetermined whether the RLF preparation timer expires, in step 707. Whenthe RLF preparation timer does not expire, the UE may wait until the RLFpreparation timer expires, in step 709.

Thereafter, when the RLF preparation timer expires, the UE determineswhether Ncell measurement can be performed, in step 711. As a result ofthe determination, when the Ncell measurement can be performed, the UEtransmits a message including information on the start of the timer T310of the MeNB link to the SeNB, together with an Ncell measurement report,in step 713. At this time, the SeNB may select an alternative cell, instep 714. Thereafter, in step 721, the UE waits for a preset time. Atthis time, it is preferable that the UE waits based on the RLFpreparation timer.

When the Ncell measurement cannot be performed, the UE transmits amessage including information on the start of the timer T310 of the MeNBlink, to the SeNB, in step 715. Thereafter, in step 721, the UE waitsfor a preset time. At this time, it is preferable that the UE waitsbased on the RLF preparation timer. At this time, the SeNB instructs theUE to perform the Ncell measurement, in step 717. In step 719, the UEreports a result of the performed Ncell measurement to the SeNB.

After step 721 or 719, the eNB determines whether there is a macro cellsuitable for a handover based on the measurement result report by theUE, in step 723. When there is no suitable macro cell, it is determinedwhether the MeNB selects at least one macro cell preconfigured as analternative cell or currently configured as an alternative cell, by theSeNB, in step 725.

When the suitable macro cell is selected in step 723 or when the MeNBselects the alternative cell as the macro cell in step 725, the MeNBtransmits a context to the macro cell and makes preparations for thehandover, in step 727. Thereafter, the MeNB waits for the expiration ofthe timer T310, in step 729. When the timer T310 expires, the MeNBinstructs the UE to hand over to the prepared macro cell, in step 731.At this time, the instruction of the handover may be made through theSeNB.

However, when the MeNB does not select the suitable alternative cell asthe macro cell, in step 725, the MeNB transmits the context to the SeNBand makes preparations for the handover, in step 733. Thereafter, theMeNB waits for the expiration of the timer T310, in step 735. When thetimer T310 expires, the MeNB instructs the UE to switch to the singleconnectivity with the SeNB, in step 737. At this time, the instructionof switching to the single connectivity may be made through the SeNB.

FIG. 8 is a flowchart illustrating a method of handling the RLF of FIG.6 when a link of the SeNB corresponds to the RLF, according to anembodiment of the present invention.

Referring to FIG. 8, in step 801, the UE determines whether the timerT310 of the SeNB link starts. When the timer T310 of the SeNB starts,the UE determines whether the timer T310 of the MeNB does not operate,in step 803. As a result of the determination, when the link of the MeNBis good, an RLF preparation timer starts, in step 805. Thereafter, theUE determines whether the RLF preparation timer expires, in step 807.When the RLF preparation timer does not expire, the UE waits until theRLF preparation timer expires, in step 809.

Thereafter, when the RLF preparation timer expires, the UE determineswhether Ncell measurement can be performed, in step 811. As a result ofthe determination, when the Ncell measurement can be performed, the UEtransmits a message including information on the start of the timer T310of the SeNB link to the MeNB, together with an Ncell measurement report,in step 813. At this time, the SeNB may select an alternative cell instep 814. Thereafter, in step 821, the UE waits for a preset time. Atthis time, it is preferable that the UE waits based on the RLFpreparation timer.

When the Ncell measurement cannot be performed, the UE transmits amessage including information on the start of the timer T310 of the MeNBlink, to the MeNB, in step 815. Thereafter, in step 821, the UE waitsfor a preset time. At this time, it is preferable that the UE waitsbased on the RLF preparation timer. At this time, the MeNB instructs theUE to perform the Ncell measurement, in step 817. In step 819, the UEreports a result of the performed Ncell measurement to the MeNB.

After step 821 or 819, the eNB determines whether there is a pico cellsuitable for a handover based on the measurement result report by theUE, in step 823. When there is no suitable pico cell, it is determinedwhether the MeNB selects at least one pico cell preconfigured as analternative cell or currently configured as an alternative cell, by theSeNB, in step 825.

When the suitable pico cell is selected in step 823 or when the MeNBselects the alternative cell as the pico cell in step 825, the MeNBtransmits a context to the selected pico cell and makes preparations forthe handover, in step 827. At this time, the same process may beperformed in the SeNB. Thereafter, the MeNB waits for the expiration ofthe timer T310, in step 829. When the timer T310 expires, the MeNBinstructs the UE to hand over to the prepared pico cell, in step 831.

However, when the MeNB does not select the suitable alternative cell asthe pico cell in step 825, the MeNB waits until the timer T310 expires,in step 833. When the timer T310 expires, the MeNB instructs the UE toswitch to the single connectivity with the SeNB, in step 835.

The embodiments of the present invention for handling the RLF describedin FIGS. 4 to 8 may be generalized by the multi-eNB configuration inwhich the UE is connected to a plurality of eNBs and one of the eNBsoperates as the MeNB. For example, in the embodiment for handling theRLF described in FIG. 6 (preparing the initial RLF), when the UE reportsthe RLF expected in one link connected to the MeNB, the MeNB may prepareanother cell to handle flows serviced by an eNB having a link in an RLFstate. An alternative cell selected by the preparation may be one cellin a set of existing cells providing a service to the UE or a new cellthat is not included in the set of cells providing the service to theUE. Similarly, the embodiment for handling the RLF described in the partrelated to FIGS. 4 and 5 may be applied to the multi-eNB configuration.

In some embodiments of the present invention, the eNB having receivedthe RLF expected message may prepare one or more cells for the handover.This can prepare many more cells for the re-establishment, and thus,increase chances that the UE selects the prepared cells for there-establishment, thereby increasing a success rate of there-establishment. In another embodiment of the present invention, theRLF expected message may be transmitted once or more.

The RLF message in the above described embodiments may be a new layer 2or 3 message, an information element in the existing message, or a layer1 level signal for a faster and stronger indication. For example, anRACH code may be given to the UE for such a purpose. Alternatively, insome embodiments of the present invention, a legacy connectionre-establishment request may be used for the RLF message. The MeNB maydetermine which link is in the RLF state based on the link through whichthe connection re-establishment request is received.

In the above described embodiments of the present invention, it has beendescribed that the RLF is triggered based on the expiration of the timerT310. However, the above described embodiments can be equally applied tothe RLF trigger by a different cause such as, for example, the RLFincluding RACH failure on the SeNB or RLC failure on the SeNB.

According to an embodiment of the present invention, the RLF indicationmay include a RLF cause value. The RLF cause may include at least one ofthe expiration of the timer T310, the failure of the RACH on the SeNB,and the failure of an RLC layer on the SeNB. The random access may besupported by one of the cells of the SeNB.

FIG. 20 is a flowchart illustrating of a method of transmitting an RLFindication including an RLF cause value, according to an embodiment ofthe present invention.

Referring to FIG. 20, a UE 2010 is dually connected to an MeNB 2020 andan SeNB 2030, in step 2051. At this time, for example, when a randomaccess fails, the SeNB 2030 cannot know of the failure. Similarly, whenthe maximum number of re-transmissions is made in a cell of the SeNB2030, the RLF is required to be triggered. The RLF is triggered in allscenarios, such as, for example, the RLF due to L1 out-of-sync, the RACHfailure of the SeNB, or the RLC failure of the SeNB during a preset timeperiod (for example, T310). Further, in order to allow the MeNB toperform a proper operation, the information is transmitted to the duallyconnected MeNB, in step 2053. Further, when the RLF is detected from thelink of the SeNB, in step 2052, the UE may stop all uplink transmission,such as CQI/SR/SRS, as soon as possible in order to prevent thegeneration of unnecessary uplink interference.

For example, an IE may be defined as follows to identify a cause of theRLF on the SeNB.

SeNB-RLF-Cause ENUM {T310 Expiry, RACH Failure, RLC Failure}

In an alternative embodiment of the present invention, the RLFindication may include a cause value. The cause may include the RACHfailure on the SeNB and the RLC failure on the SeNB.

For example, an IE may be defined as follows to identify a cause of theRLF on the SeNB.

SeNB-RLF-Cause ENUM {RACH Failure, RLC Failure}

In another alternative embodiment of the present invention, when theRLFs are simultaneously generated, SeNB-RLF-Cause IE may expressmultiple types of the RLF. For example, an IE may be defined as followsto identify a cause of the RLF on the SeNB.

SeNB-RLF-Cause ENUM {T310 Expiry, RACH Failure, RLC Failure, T310 Expiry& RACH Failure, RACH Failure & RLC Failure, T310 Expiry & RLC Failure,T310 Expiry & RACH Failure & RLC Failure}

In such a case, in some embodiments of the present invention,SeNB-RLF-Cause IE itself may be considered as the RLF indication.

In some embodiments of the present invention, the IE may be includedwithin the existing RRC message. Alternatively, a new RRC message may bemade to carry the IE.

FIG. 21 is a flowchart illustrating a method of transmitting an RLFindication included in a measurement report configuration, according toan embodiment of the present invention.

Referring to FIG. 21, a UE 2110 is dually connected to an MeNB 2120 andan SeNB 2130, in step 2151. At this time, when the UE 2110 detects theRLF from a cell of the SeNB 2130, in step 2153, the UE 2110 may insertcause information (for example, SeNB-RLF-cause IE) of the RLF into ameasurement report and transmit the measurement report to the MeNB 2120,in step 2154. At this time, the MeNB 2120 may retransmit a message for ameasurement configuration to report the RLF of the SeNB 2130 to the UE2110, in step 2152.

Meanwhile, in some embodiments of the present invention, a new expandedmeasurement report may be generated to include the cause information ofthe RLF. In this case, when the UE 2110 detects the RLC from the cell ofthe SeNB 2130 in step 2153, the UE 2110 may transmit the expandedmeasurement report including SeNB-RLF-Cause IE that indicates the causeof the RLF to the MeNB 2120, in step 2154. At this time, when the UE2110 detects the RLF from the cell of the SeNB, the expanded measurementreport may be configured to be transmitted by the (master) eNB, in step2152.

FIG. 22 is a flowchart illustrating a method of transmitting an RLFindication included in a measurement report configuration, according toan embodiment of the present invention.

Referring to FIG. 22, a UE 2210 is dually connected to an MeNB 2220 andan SeNB 2230, in step 2251. In this case, when the UE 2210 detects theRLF from a cell of the SeNB 2230, in step 2252, the UE 2210 may alwaystransmit an expanded measurement report to the MeNB 2220, in step 2253.The embodiment related to FIG. 22 does not require the measurementconfiguration, unlike the embodiment described in the part related toFIG. 21. When the UE 2210 detects the RLC from the cell of the SeNB2230, in step 2252, the UE 2210 may prepare the expanded measurementreport including information on the cause of the RLF (for example,SeNB-RLF-Cause IE). At this time, in some embodiments of the presentinvention, the expanded measurement report may include measurementresults of a serving cell and neighboring cells if possible. Further,the UE 2210 may transmit the expanded measurement report to the MeNB2220, in step 2253.

Meanwhile, in some embodiments of the present invention, anRRCConnectionReestablishmentRequest may be used to indicate the RLF ofthe SeNB and the failure cause thereof. At this time, a spare field ofReestablishmentCause may be used as a signal of the RLF indication ofthe SeNB. A detailed description thereof is set forth below. As soon asthe MeNB receives the RRCConnectionReestablishmentRequest, the MeNB maydetermine whether the RRCConnectionReestablishmentRequest is madeaccording to the RLC of the SeNB based on the ReestablishmentCause.Further, the MeNB may perform a proper operation instead of processingthe RRCConnectionReestablishmentRequest message according to a legacymanner. Alternatively, when a physCellId field corresponding to a PUCCHcorresponds to the cell of the SeNB, the MeNB may use a re-establishmentrequest to indicate the RLF of the SeNB. In another embodiment of thepresent invention, the SeNB-RLF-Cause IE may be included in theRRCConnectionReestablishmentRequest to clearly indicate the cause of theRLF of the SeNB. Only when the RRCConnectionReestablishmentRequest isused to indicate the RLF of the SeNB, the IE may be included in theRRCConnectionReestablishmentRequest.

For example, messages may be configured as shown in Table 1 below.

TABLE 1 -- ASN1START RRCConnectionReestablishmentRequest ::= SEQUENCE {criticalExtensions CHOICE { rrcConnectionReestablishmentRequest-r8RRCConnectionReestablishmentRequest-r8-IEs, criticalExtensionsFutureSEQUENCE { } } } RRCConnectionReestablishmentRequest-r8-IEs ::= SEQUENCE{ ue-Identity ReestabUE-Identity, reestablishmentCauseReestablishmentCause, spare BIT STRING (SIZE (2)) } ReestabUE-Identity::= SEQUENCE { c-RNTI C-RNTI, physCellId PhysCellId, shortMAC-IShortMAC-I } ReestablishmentCause ::= ENUMERATED {reconfigurationFailure, handoverFailure, otherFailure, SeNB-RLF} --ASN1STOP

Meanwhile, in another embodiment of the present invention, the UE mayinform the MeNB of an RLF state generated in the SeNB by transmitting aUE assistance information (UEAssistanceInformation) message to the MeNB.At this time, the UE assistance information message, which the UEtransmits to the MeNB, may include one or more of a cause of the RLFgenerated in the SeNB, that is, the physical layer failure, the RACHfailure, and the RLC failure.

The UE may perform an RLM operation on at least one cell of the SeNB inwhich an uplink control channel, that is, a PUCCH, is configured. TheRLM operation on the cell of the SeNB may be performed in the same wayas that of the RLM operation on the MeNB in which the PUCCH isconfigured, that is, a PCell. Further, at this time, parameters such asT310, N310, N311 and the like may be used. When the UE performs the RLMoperation, and thus, detects the RLF of the SeNB, the UE may stop uplinktransmission for all cells of the corresponding SeNB and inform the MeNBthat the RLF is generated in the SeNB by transmitting a UE assistanceinformation message to the MeNB. The UE assistance information a messagemay include a cause of the RLF generated in the SeNB. Specifically, theUE assistance information message may be as shown in Table 2 below.

TABLE 2 UEAssistanceInformation message -- ASN1STARTUEAssistanceInformation-r11 ::= SEQUENCE { criticalExtensions CHOICE {c1 CHOICE { ueAssistanceInformation-r11 UEAssistanceInformation-r11-IEs,spare3 NULL, spare2 NULL, spare1 NULL }, criticalExtensionsFutureSEQUENCE { } } } UEAssistanceInformation-r11-IEs ::= SEQUENCE {powerPrefIndication-r11 ENUMERATED {normal,lowPowerConsumption} OPTIONAL, lateNonCriticalExtension OCTET STRINGOPTIONAL, nonCriticalExtension UEAssistanceInformation-r12-IEs OPTIONAL} UEAssistanceInformation-r12-IEs ::= SEQUENCE { SCGRLFCause ENUMERATED{ t310-Expiry, randomAccessProblem, rlc-MaxNumRetx, spare1} OPTIONAL,lateNonCriticalExtension OCTET STRING OPTIONAL, nonCriticalExtensionSEQUENCE { } OPTIONAL } -- ASN1STOP UEAssistanceInformation fielddescriptions powerPrefIndication Value lowPowerConsumption indicates theUE prefers a configuration that is primarily optimised for power saving.Otherwise the value is set to normal. SCGRLFCause This field is used toindicate the cause of the last radio link failure that was detected.onPScell.

FIGS. 23 to 25 are diagrams illustrating MAC CE, according to anembodiment of the present invention.

In some embodiments of the present invention, an RLF indication having acause value of the RLF may be defined by a new message or a new MAC CE.Alternatively, the RLF indication having the cause value of the RLF maybe added as a new information element within the existing message. Anidentifier of a cell may be identified together with the RLF indication.When the RLF indication does not include the cell identifier, it may beconsidered to indicate the RLF in the cell of the SeNB (for example,PUCCH cell of the SeNB). For example, a new MAC CE illustrated in FIGS.23 and 24 may be defined.

Referring to FIG. 23, C7 of the new MAC CE indicates the RLF due to theRACH failure on a PUCCH carrying cell of the SeNB, and C6 indicates theRLF due to the RLC failure on a PUCCH carrying cell of the SeNB.Further, other bits may be in a reserved state.

Referring to FIG. 24, in order to indicate the cause of the RLF due tothe RACH failure and the RLC failure, the MAC CE may be defined. At thistime, C7 of the MAC CE indicates the RLF due to L1 out-of-sync on thePUCCH carrying cell of the SeNB, C6 indicates the RLF due to the RACHfailure on the PUCCH carrying cell of the SeNB, and C5 indicates the RLFdue to the RLC failure on the PUCCH carrying cell of the SeNB. Further,other bits may be in a reserved state.

Referring to FIG. 25, according to an embodiment of the presentinvention, the RLF may be transmitted to the MeNB by using the existingmessage (VarRLF-Report). At this time, in some embodiments of thepresent invention, “failedPCellId” may be set to identify the cell ofthe MeNB or the SeNB in which the RLF is generated. For example, the“failedPCellId” may be set as a PCell (of the MeNB) or a pSCell(corresponding to a special cell having the PUCCH of the SeNB).“rlf-Cause” may be set as one of the above described causes.Availability of the stored RLF report may be identified by the MeNBmaking a request for transmitting the stored RLF report to the UE. In anembodiment of the present invention, an indication of the availabilitytransmitted to the MeNB may indicate that the RLF report is for theSeNB. For example, the availability may be indicated by transmitting theMAC CE. At this time, C7, C6, and C5 of the MAC CE may transmit theabove-described information. That is, C7 indicates the RLF due to L1out-of-sync on the PUCCH carrying cell of the SeNB, C6 indicates the RLFdue to the RACH failure on the PUCCH carrying cell of the SeNB, and C5indicates the RLF due to the RLC failure on the PUCCH carrying cell ofthe SeNB. Alternatively, as illustrated in FIG. 23, C7 of the new MAC CEmay indicate the RLF due to the RACH failure on a PUCCH carrying cell ofthe SeNB and C6 may indicate the RLF due to the RLC failure on the PUCCHcarrying cell of the SeNB. Meanwhile, C0 may indicate the availabilityof the RLF report. The eNB may use a legacy UE information requestmessage and a UE information response message to acquire the report fromthe UE.

In another embodiment of the present invention, a physical (PHY) layersignal may be used to indicate the RLF on the SeNB and a cause thereof.For example, an RACH code may be preconfigured to specify the cause ofthe RLF.

In an embodiment of the present invention, the timer T310 for detectingthe RLF may be configured to have different values.

One or more causes of the RLF may be applied to one or more cells servedby the SeNB. For example, the RACH may be supported in only one cell ofthe SeNB (called pSCell), and thus, the RACH failure may be triggeredonly on the pSCell.

Further, in some embodiments of the present invention, an RLF messagemay include link identification information of a link in which the RLFis generated. The link identification information may indicate whetherthe link is a link of the MeNB or a link of the SeNB. For example, thelink identification information may be a one bit identifier in the dualconnectivity. The link identification information may be expanded in amulti-eNB system. FIG. 9 is a flowchart illustrating a process of anearly indication of the RLF in a single connectivity system.

Referring to FIG. 9, an early indication of the RLF before the RLF timerexpires may be used in the single connectivity system. A UE 910 expectsthe RLF in a first link, in step 951. For example, when the RLF timerT310 starts, the RLF may be expected. Alternatively, after a presettimer elapses from the start of the timer T310, the RLF may be expected.A detailed description thereof is set forth above.

Thereafter, in step 952, the UE 910 transmits an RLF expected message toa source eNB 920. When the source eNB 920 successfully receives the RLFexpected message, the source eNB 920 sufficiently prepares a target cell930 for early re-establishment. Steps 952 to 955 are similar to theoperations of the above-described embodiments.

Thereafter, since the new cell 930, which has been already prepared forthe re-establishment, (having the user context) has a high change whenthe RLF is actually generated, and thus, the UE 910 attempts there-establishment to the new cell 930, a success rate of there-establishment may increase.

The method of handling the RLF in the dual connectivity or the singleconnectivity has been described above.

Switching between the single connectivity and the dual connectivity isdescribed in greater detail below.

FIG. 10 is a flowchart illustrating a method of switching to the singleconnectivity from the dual connectivity, according to an embodiment ofthe present invention.

Referring to FIG. 10, a UE 1010 is currently connected to an MeNB 1020and an SeNB 1030 in a dual connectivity mode. Further, the UE 1010operates to switch to the single connectivity with the MeNB 1020. The UE1010 determines the switching to the single connectivity mode, in step1051, and transmits a single connectivity switching request message,indicating that the UE 1010 desires to switch to the single connectivitymode, to the MeNB 1020, in step 1052. At this time, the singleconnectivity switching request message may include an ID of the MeNB1020.

Thereafter, the MeNB 1020 may fetch information on flows (for example,QoS and the like) serviced by the SeNB 1030. Specifically, in step 1053,the MeNB 1020 determines to acquire flow information of another eNB (forexample, the SeNB 1030), which the MeNB 1020 does not have. Thereafter,in step 1054, the MeNB 1020 transmits a request message of flowinformation serviced by the SeNB 1030, to the SeNB 1030. The SeNB 1030transmits a response message of the flow information serviced by theSeNB 1030, to the MeNB 1020, in step 1055.

The MeNB 1020 performs an acceptance control of the flows based on thereceived flow information, in step 1056. When the flows cannot besupported, the MeNB 1020 may downgrade or terminate the flows, in step1056-1.

Thereafter, in order to identify the service provided to the UE 1010 inthe single connectivity mode, the MeNB 1020 transmits a singleconnectivity indication message to the SeNB 1030, in step 1057. The SeNB1030, having received the single connectivity indication message,deletes the UE context, in step 1058. At this time, in some embodimentsof the present invention, the SeNB 1030 may delete the UE context aftera preset time elapses.

Further, the MeNB 1020 triggers the switching to the singleconnectivity, in step 1059, and transmits changed flow information and asingle connectivity switching response message to the UE 1010, in step1060, so as to operate the UE 1010 to perform the switching to thesingle connectivity. Thereafter, in step 1061, the UE 1010 deletes thecontext of the SeNB 1030, and transmits the changed flows to the MeNB1020, so as to switch to the single connectivity mode with the MeNB1020.

FIG. 11 is a flowchart illustrating a method of switching to the singleconnectivity from the dual connectivity, according to another embodimentof the present invention.

Referring to FIG. 11, a UE 1110 is currently connected to an MeNB 1120and an SeNB 1130 in a dual connectivity mode. Further, the UE 1130operates to switch to the single connectivity with the SeNB 1130. The UE1110 determines the switching to the single connectivity mode, in step1151, and transmits a single connectivity switching request messageindicating that the UE 1110 desires to switch to the single connectivitymode, to the MeNB 1020, in step 1152. At this time, the singleconnectivity switching request message may include an ID of the SeNB1130.

Thereafter, the MeNB 1120 determines to provide information on flows(for example, QoS and the like) serviced by the MeNB 1120, to the SeNB1130, in step 1153. In step 1154, the MeNB 1120 transmits the singleconnectivity request message to the SeNB 1130. At this time, the MeNB1120 may also transmit information on flows serviced by the MeNB 1120.

The SeNB 1130 performs an acceptance control of the flows based on thereceived flow information, in step 1155. When the flows cannot besupported, the MeNB 1130 may downgrade or terminate the flows, in step1156.

Thereafter, in order to identify the service provided to the UE 1010 inthe single connectivity mode, the SeNB 1130 transmits a singleconnectivity response message to the MeNB 1030, in step 1157. At thistime, flow information may be transmitted to the MeNB 1120 together withthe single connectivity response message.

The MeNB 1120, having received the single connectivity response message,deletes the UE context, in step 1158. At this time, in some embodimentsof the present invention, the MeNB 1120 may delete the UE context aftera preset time elapses.

Further, the MeNB 1120 triggers the switching to the singleconnectivity, in step 1159, and transmits changed flow information and asingle connectivity switching response message to the UE 1110, in step1160, so as to operate the UE 1110 to perform the switching to thesingle connectivity. Thereafter, in step 1161, the UE 1110 deletes thecontext of the MeNB 1120, and transmits the changed flows to the SeNB1130, so as to switch to the single connectivity mode with the SeNB1130.

FIG. 12 is a flowchart illustrating a method of switching to the singleconnectivity from the dual connectivity, according to another embodimentof the present invention.

Referring to FIG. 12, a UE 1210 is currently connected to an MeNB 1220and an SeNB 1230 in a dual connectivity mode. Further, the UE 1210operates to switch to the single connectivity with the MeNB 1220. Instep 1251, the MeNB 1220 determines that the UE 1210 switches to thesingle connectivity mode with the MeNB 1220.

Thereafter, the MeNB 1220 fetches information on flows (for example, QoSand the like) serviced by the SeNB 1230. Specifically, in step 1252, theMeNB 1220 determines to acquire flow information of another eNB (forexample, the SeNB 1230), which the MeNB 1220 does not have. Thereafter,in step 1253, the MeNB 1220 transmits a request message of flowinformation serviced by the SeNB 1230, to the SeNB 1230. The SeNB 1230transmits a response message of the flow information serviced by theSeNB 1220, to the MeNB 1020, in step 1254.

The MeNB 1220 performs an acceptance control of the flows based on thereceived flow information, in step 1255. When the flows cannot besupported, the MeNB 1220 may downgrade or terminate the flows, in step1256.

Thereafter, in order to identify the service provided to the UE 1210 inthe single connectivity mode, the MeNB 1220 transmits a singleconnectivity indication message to the SeNB 1230, in step 1257. The SeNB1230 having received the single connectivity indication message deletesthe UE context, in step 1258. At this time, in some embodiments of thepresent invention, the SeNB 1230 may delete the UE context after apreset time elapses.

Further, the MeNB 1220 triggers the switching to the singleconnectivity, in step 1259, and transmits an ID of the MeNB 1220,changed flow information, and a single connectivity switching message,to the UE 1210, in step 1260, so as to operate the UE 1210 to performthe switching to the single connectivity. Thereafter, in step 1261, theUE 1210 deletes the context of the SeNB 1230, and transmits the changedflows to the MeNB 1220, so as to switch to the single connectivity modewith the MeNB 1220.

FIG. 13 is a flowchart illustrating a method of switching to the singleconnectivity from the dual connectivity, according to another embodimentof the present invention.

Referring to FIG. 13, a UE 1310 is currently connected to an MeNB 1320and an SeNB 1330 in a dual connectivity mode. Further, the UE 1310operates to switch to the single connectivity with the SeNB 1330. Instep 1351, the MeNB 1320 determines that the UE 1310 switches to thesingle connectivity mode with the SeNB 1330.

Thereafter, the MeNB 1320 determines to provide information on flows(for example, QoS and the like) serviced by the MeNB 1320, to the SeNB1330, in step 1352. In step 1353, the MeNB 1320 transmits the singleconnectivity switching request message to the SeNB 1330. At this time,the MeNB 1320 may also transmit information on flows serviced by theMeNB 1320.

The SeNB 1330 performs an acceptance control of the flows based on thereceived flow information, in step 1354. When the flows cannot besupported, the SeNB 1330 may downgrade or terminate the flows, in step1355.

Thereafter, in order to identify the service provided to the UE 1310 inthe single connectivity mode, the SeNB 1330 transmits a singleconnectivity response message to the MeNB 1320, in step 1356. At thistime, changed flow information may be transmitted to the MeNB 1320together with the single connectivity response message.

Further, the MeNB 1320 triggers the switching to the singleconnectivity, in step 1357, and transmits an ID of the MeNB 1320, thechanged flow information, and a single connectivity switching responsemessage to the UE 1310, in step 1358, so as to operate the UE 1310 toperform the switching to the single connectivity.

Thereafter, the MeNB 1320 deletes the UE context, in step 1359. At thistime, in some embodiments, the MeNB 1320 may delete the UE context aftera preset time elapses.

Thereafter, in step 1360, the UE 1310 deletes the context of the MeNB1320, and transmits the changed flows to the SeNB 1330, so as to switchto the single connectivity mode with the SeNB 1330.

FIG. 14 is a flowchart of a method of switching to the dual connectivityfrom the single connectivity, according to an embodiment of the presentinvention.

Referring to FIG. 14, a UE 1410 currently operates in the singleconnectivity mode with an MeNB 1420. Further, the UE 1410 determines theswitching to the dual connectivity from the signal connectivity, in step1451, and transmits a dual connectivity switching request message,indicating that the UE 1410 desires to switch to the dual connectivitymode, to the MeNB 1420, in step 1452. At this time, the dualconnectivity switching request message may include an ID of an SeNB1430.

The MeNB 1420 identifies the dual connectivity with the SeNB 1430, instep 1453. For example, the identification of the dual connectivity maybe performed by identifying a flow acceptance control and a pair of theMeNB and the SeNB to support the dual connectivity, but is not limitedthereto.

Thereafter, the MeNB 1420 transmits the dual connectivity requestmessage to the SeNB 1430, in step 1454. At this time, the dualconnectivity request message may include flow information. The SeNB 1430performs an acceptance control of the flows based on the received flowinformation, in step 1455. When the flows cannot be supported, the SeNB1430 may downgrade or terminate the flows, in step 1456.

In step 1457, the SeNB 1430 identifies support for the acceptance of thedual connectivity by transmitting a dual connectivity response messageto the MeNB 1420. Further, in step 1458, the MeNB 1420 triggers theswitching to the dual connectivity. Thereafter, in step 1459, the MeNB1420 transmits the dual connectivity switching response message to theUE 1410 to operate the UE 1410 to perform the switching to 2 o the dualconnectivity. In step 1460, the UE 1410 switches to the dualconnectivity mode with the MeNB 1420 and the SeNB 1430.

FIG. 15 is a flowchart illustrating a method of switching to the dualconnectivity from the single connectivity, according to anotherembodiment of the present invention.

Referring to FIG. 15, a UE 1510 currently operates in the singleconnectivity mode with an MeNB 1520. Further, in step 1551, the MeNB1520 triggers the switching to the dual connectivity from the singleconnectivity. At this time, the MeNB 1520 identifies the dualconnectivity with the SeNB 1530. For example, the identification of thedual connectivity may be performed by identifying a flow acceptancecontrol and a pair of the MeNB and the SeNB to support the dualconnectivity, but is not limited thereto.

Thereafter, in step 1552, the MeNB 1520 transmits a dual connectivityrequest message, indicating that the UE 1510 desires to switch to thedual connectivity mode, to the SeNB 1530. At this time, the dualconnectivity request message may include flow information.

The SeNB 1530 performs an acceptance control of the flows based on thereceived flow information, in step 1553. When the flows cannot besupported, the SeNB 1530 may downgrade or terminate the flows, in step1555.

In step 1554, the SeNB 1530 identifies support for the acceptance of thedual connectivity by transmitting a dual connectivity response messageto the MeNB 1520. Further, in step 1556, the MeNB 1520 triggers theswitching to the dual connectivity. Thereafter, in step 1557, the MeNB1520 operates the UE 1510 to perform the switching to the dualconnectivity by transmitting the dual connectivity switching responsemessage, including an ID of the SeNB 1530 and flow information, to theUE 1510. In step 1558, the UE 1510 switches to the dual connectivitymode with the MeNB 1520 and the SeNB 1530. Further, the UE 1510transmits the dual connectivity response message to the MeNB 1520, instep 1559.

FIG. 16 is a block diagram illustrating the UE, according to anembodiment of the present invention.

Referring to FIG. 16, a controller 1610 controls the UE to perform anyoperation described in the aforementioned embodiments of the presentinvention. For example, the controller 1610 may provide a control totransmit an RLF expected message related to a first link to the eNB,transmit an RLF message related to the first link to the eNB through asecond link, and receive an instruction of a handover to a target cellselected by the eNB according to the RLF expected message.

A communication unit 1620 transmits and receives a signal according toany operation described in the aforementioned embodiments of the presentinvention. For example, the communication unit 1620 may transmit the RLFmessage and the RLF expected message to the MeNB.

FIG. 17 is a block diagram illustrating the eNB, according to anembodiment of the present invention.

Referring to FIG. 17, a controller 1710 controls the eNB to perform anyone operation described in the aforementioned embodiments of the presentinvention. For example, the controller 1710 may provide a control toreceive an RLF expected message related to a first link from the UE,search for a target cell for a handover of the UE according to the RLFexpected message, select the target cell when the target cell exists asa result of the search, receive an RLF message related to the first linkfrom the UE through a second link, and transmit an instruction of ahandover to the selected target cell to the UE.

A communication unit 1720 transmits and receives a signal according toany operation described in the aforementioned embodiments of the presentinvention. For example, the communication unit 1720 may receive the RLFmessage and the RLF expected message from the UE.

Software components including instructions or code for performing themethodologies described herein may be stored in one or more associatedmemory devices (e.g., Read Only Memory (ROM), fixed or removable memory)and, when ready to be utilized, loaded in part or in whole (e.g., intoRandom Access Memory (RAM)) and executed by the controller.

While the invention has been shown and described with reference tocertain embodiments thereof, it will be understood by those skilled inthe art that various changes in form and detail may be made thereinwithout departing from the spirit and scope of the invention as definedby the appended claims.

What is claimed is:
 1. A method by a terminal in a wirelesscommunication system supporting dual connectivity of a first basestation and a second base station, the method comprising: receivinginformation on a number of consecutive out-of-syncs for a special cell;starting a timer for the special cell upon receiving the information onthe number of consecutive out-of-syncs; detecting a radio link failure(RLF) for the special cell upon an expiration of the timer; andtransmitting, to the first base station, a failure message includingfailure type in case that the RLF for the special cell is associatedwith the second base station, wherein the failure type indicates anexpiration of the timer.
 2. The method of claim 1, wherein transmittingthe failure message further comprises: suspending a transmission for atleast one bearer associated with a cell of the second base station. 3.The method of claim 1, wherein a physical uplink control channel (PUCCH)for the second base station is transmitted on a primary secondary cellassociated with the second base station.
 4. The method of claim 1,wherein the RLF is detected based on a random access problem indication,and wherein the failure type indicates the random access problem.
 5. Themethod of claim 1, wherein the RLF is detected in case that a radio linkcontrol (RLC) maximum number retransmission has been reached, andwherein the failure type indicates the RLC maximum numberretransmission.
 6. A method by a first base station in a wirelesscommunication system supporting dual connectivity of the first basestation and a second base station, the method comprising: transmitting,to a terminal, a message for configuring a cell associated with thefirst base station; and receiving, from the terminal, a failure messageincluding failure type in case that a radio link failure (RLF) for aspecial cell is detected upon an expiration of a timer for the specialcell and the special cell is associated with the second base station,wherein the timer is started in case that information on a number ofconsecutive out-of-syncs for the special cell is received at theterminal, and wherein the failure type indicates an expiration of thetimer.
 7. The method of claim 6, wherein a transmission for at least onebearer associated with a cell of the second base station is suspended.8. The method of claim 6, wherein a physical uplink control channel(PUCCH) for the second base station is received on a primary secondarycell associated with the second base station.
 9. The method of claim 6,wherein the RLF is detected based on a random access problem indication,and wherein the failure type indicates the random access problem. 10.The method of claim 6, wherein the RLF is detected in case that a radiolink control (RLC) maximum number retransmission has been reached, andwherein the failure type indicates the RLC maximum numberretransmission.
 11. A terminal in a wireless communication systemsupporting dual connectivity of a first base station and a second basestation, the terminal comprising: a transceiver; and a controllercoupled with the transceiver and configured to: receive information on anumber of consecutive out-of-syncs for a special cell, start a timer forthe special cell upon receiving the information on the number ofconsecutive out-of-syncs, detect a radio link failure (RLF) for thespecial cell upon an expiration of the timer, and transmit, to the firstbase station, a failure message including failure type in case that theRLF for the special cell is associated with the second base station,wherein the failure type indicates an expiration of the timer.
 12. Theterminal of claim 11, wherein the controller is configured to suspend atransmission for at least one bearer associated with a cell of thesecond base station.
 13. The terminal of claim 11, wherein a physicaluplink control channel (PUCCH) for the second base station istransmitted on a primary secondary cell associated with the second basestation.
 14. The terminal of claim 11, wherein the RLF is detected basedon a random access problem indication, and wherein the failure typeindicates the random access problem.
 15. The terminal of claim 11,wherein the RLF is detected in case that a radio link control (RLC)maximum number retransmission has been reached, and wherein the failuretype indicates the RLC maximum number retransmission.
 16. A first basestation in a wireless communication system supporting dual connectivity,the first base station comprising: a transceiver; and a controllercoupled with the transceiver and configured to: transmit, to a terminal,a message for configuring cell associated with the first base station,and receive, from the terminal, a failure message including failure typein case that a radio link failure (RLF) for a special cell is detectedupon an expiration of a timer for the special cell and the special cellis associated with a second base station, wherein the timer is startedin case that information on a number of consecutive out-of-syncs for thespecial cell is received at the terminal, and wherein the failure typeindicates an expiration of the timer.
 17. The first base station ofclaim 16, wherein a transmission for at least one bearer associated witha cell of the second base station is suspended.
 18. The first basestation of claim 16, wherein a physical uplink control channel (PUCCH)for the second base station is received on a primary secondary cellassociated with the second base station.
 19. The first base station ofclaim 16, wherein the RLF is detected based on a random access problemindication, and wherein the failure type indicates the random accessproblem.
 20. The first base station of claim 16, wherein the RLF isdetected in case that a radio link control (RLC) maximum numberretransmission has been reached, and wherein the failure type indicatesthe RLC maximum number retransmission.